The present invention provides high efficiency phosphorescent organic light emitting devices. The present invention relates, for example, to an organic light emitting device (OLED) over a substrate, where the OLED has an anode, a hole transporting layer (HTL), a first electron transporting layer (ETL) that is doped with a phosphorescent material, a second electron transporting layer (ETL), and a cathode. Specific embodiments of the present invention include use of an aryl-substituted oxadiazole, an aryl-substituted triazole, an aryl-substituted phenanthroline, a benzoxazole or benzthiazole compound as the first ETL that is used as the host for the emissive phosphorescent dopant material. Another embodiment comprises a second ETL that functions as a hole blocking layer between the phosphorescent doped ETL and the cathode.
Organic light emitting devices (OLEDs) include several organic layers in which at least one of the layers includes an organic material that can be made to electroluminesce by applying a voltage across the device, C. W. Tang et al., Appi. Phys. Lett. 1987, 51, 913. Certain OLEDs have been shown to have sufficient brightness, range of color and operating lifetimes for use as a practical alternative technology to LCD-based full color flat-panel displays (S. R. Forrest, P. E. Burrows and M. E. Thompson, Laser Focus World, February 1995). Since many of the thin organic films used in such devices are transparent in the visible spectral region, they allow for the realization of a completely new type of display pixel in which red (R), green (G), and blue (B) emitting OLEDs are placed in a vertically stacked geometry to provide a simple fabrication process, a small R-G-B pixel size, and a large fill factor, U.S. Pat. No. 5,707,745.
A transparent OLED (TOLED), which represents a significant step toward realizing high resolution, independently addressable stacked R-G-B pixels, was disclosed in U.S. Pat. No. 5,703,436, in which the TOLED had greater than 71% transparency when turned off and emitted light from both top and bottom device surfaces with high efficiency (approaching 1% quantum efficiency) when the device was turned on. The TOLED used transparent indium tin oxide (ITO) as the hole-injecting electrode and a Mgxe2x80x94Ag-ITO electrode layer for electron-injection. A device was disclosed in which the ITO side of the Mgxe2x80x94Ag-ITO layer was used as a hole-injecting contact for a second, different color-emitting OLED stacked on top of the TOLED. Each layer in the stacked OLED (SOLED) was independently addressable and emitted its own characteristic color. This colored emission could be transmitted through the adjacently stacked, transparent, independently addressable, organic layer or layers, the transparent contacts and the glass substrate, thus allowing the device to emit any color that could be produced by varying the relative output of the red and blue color-emitting layers.
U.S. Pat. No. 5,707,745 disclosed an integrated SOLED for which both intensity and color could be independently varied and controlled with external power supplies in a color tunable display device. U.S. Pat. No. 5,707,745, thus, illustrates a principle for achieving integrated, full color pixels that provide high image resolution, which is made possible by the compact pixel size. Furthermore, relatively low cost fabrication techniques, as compared with prior art methods, may be utilized for making such devices. Until recently, it was not believed that organic materials could be used to produce efficient room temperature electrophosphorescence. In contrast, use of fluorescent dyes in OLEDs has been known for much longer, (C. H. Chen, J. Shi, and C. W. Tang, xe2x80x9cRecent developments in molecular organic electroluminescent materials,xe2x80x9d Macromolecular Symposia, 1997, 125, 1-48; U. Brackmann, Lambdachrome Laser Dyes (Lambda Physik, Gottingen, 1997, and references cited therein) and fluorescent efficiencies in solution approaching 100% are not uncommon. (C. H. Chen, 1997, op. cit.) Fluorescence is also not affected by triplet-triplet annihilation, which degrades phosphorescent emission at high excitation densities. (M. A. Baldo, et al., xe2x80x9cHigh efficiency phosphorescent emission from organic electroluminescent devices,xe2x80x9d Nature, 1998, 395, 151-154). Consequently, fluorescent materials are suited to many electroluminescent applications, particularly passive matrix displays.
An advantage of phosphorescence is that all excitons (formed by the recombination of holes and electrons in an ETL), which are in fact predominantly triplets in an OLED, may participate in energy transfer and luminescence in certain electroluminescent materials. In contrast, only a small percentage of excitons in fluorescent emitting devices, which are singlet-based, result in fluorescent luminescence. Fluorescence is, thus, at best only one-third as efficient as phosphorescence due to the formation of three times more triplet excitons than singlet excitons. Recently it was discovered that very high efficiency organic light emitting devices could be fabricated based on electrophosphorescence (M. A. Baldo, D. F. O""Brien, M. E. Thompson and S. R. Forrest, Very high-efficiency green organic light-emitting devices based on electrophosphorescence, Applied Physics Letters, 1999, 75, 4-6.) The terms fluorescence and phosphorescence refer to the type of radiative emission, respectively, that is typically understood by one skilled in the art. That is, fluorescence refers to radiative emission from an excited singlet state and phosphorescence refers to radiative emission from an excited triplet state.
In view of the improved external efficiency that can be realized for electrophosphorescent OLEDs, it would be desirable to find additional materials as host materials for emissive phosphorescent dopant materials so that even higher external efficiencies can be produced.
The OLEDs of the present invention typically have an anode, an HTL, a first ETL that is doped with a phosphorescent material, a second ETL, and a cathode. The OLED may be formed over the substrate with either the anode side closest to a substrate or the cathode side closest to the substrate. When the cathode side is closest to the substrate the OLED is considered to be inverted. In each embodiment, the first ETL, which is doped with the phosphorescent material, is positioned between an HTL and the second ETL.
The present invention also encompasses stacked OLED structures, in which at least one OLED or inverted OLED has a doped first ETL between an HTL and a second ETL. When more than one OLED is stacked over the substrate, the OLEDs are stacked one upon the other.
The present invention is directed, in particular, to an ETL host material comprised of an aryl-substituted oxadiazole, an aryl-substituted triazole, an aryl-substituted phenanthroline, a benzoxazole or benxthiazole compound. As a representative embodiment of the present invention, the emissive phosphorescent dopant material may be fac-tris (2-phenylpyridine) iridium (Ir(ppy)3), having the chemical formula: 
More specifically, the phosphorescent-host material may be comprised of an aryl-substituted oxadiazole, such as represented by the chemical formula: 
As yet another embodiment of the present invention, the phosphorescent-host material may be comprised of an aryl-substituted triazole, for example, such as 3-phenyl-4-(1-naphthyl)-5-phenyl-1,2,4-triazole (TAZ) as represented by the following chemical formula: 
As yet still another embodiment of the present invention, the phosphorescent-host material may be comprised of an aryl-substituted phenanthroline, for example, such as 2,9-dimethyl-4,7-diphenyl-phenanthroline (BCP), as represented by the following chemical formula: 
Without intending to be limited to the theory of how the high efficiencies of the devices of the present invention works, it is believed that the high efficiencies that are achieved for the representative embodiments of the present invention, Ir(ppy)3 in OXD-7, TAZ, BCP, or a zinc benzoxazole compound are due to the fact that the host matrix comprises a host material having a triplet state energy sufficiently higher than the triplet state energy of the emissive phosphorescent dopant that very little, if any energy transfer of the triplet energy from the emissive dopant to the host material occurs. Such energy transfer of the triplet excitation energy from the emissive dopant to the host material can result in significant non-radiative energy losses.
While the rate and extent of energy transfer from the triplet dopant to the triplet may depend on many factors, including, for example, the relative radiative lifetimes of the respective triplet states, it is believed than an energy difference of at least about 0.1 eV between the triplet host and triplet dopant is sufficient to prevent significant non-radiative energy losses to the host.
As representative embodiments of the present invention, the host matrix may be comprised of an aryl-substituted oxadiazole, an aryl-substituted triazole, an aryl-substituted phenanthroline, a benzoxazole or a benzthiazole compound that has a lowest triplet state with an energy greater than the emissive dopant triplet energy.
As still another aspect of the present invention, it is believed that the improved external efficiency of the devices of the present invention may also be due, in part, to the improved recombination efficiency of holes and electrons in phosphorescent dopant materials that is provided by charge-carrier-trapping of holes. A phosphorescent dopant material that is capable of trapping holes is one for which the HOMO energy of the phosphorescent dopant molecules is less than the ionization potential of the host molecules. Thus, the present invention is further directed to phosphorescent-doped hosts wherein the phosphorescent molecules are capable of improving the hole/electron recombination efficiency by functioning as charge carrier trapping sites, in particular, charge carrier trapping sites for trapping holes.
The OLEDs of the present invention are directed, in particular, to devices that include an emissive layer comprised of a host material having a lowest triplet state having an energy level that is larger than the triplet state energy level of the phosphorescent dopant material.
The present invention is further directed to a phosphorescent dopant material comprised of molecules having charge carrier trapping sites. As a representative embodiment of the present invention, the emissive layer may be comprised of a host electron transporting material and a phosphorescent dopant material that traps holes in the highest occupied molecular orbitals (HOMO) of the phosphorescent dopant molecule. Such phosphorescent dopant-materials have a HOMO energy that is less than the ionization potential of the host electron transporting level. In addition, such phosphorescent dopant molecules have a lowest unoccupied molecular orbital (LUMO) energy level that is lower than the LUMO energy level of the host material.
While the present invention has been illustrated herein for OLEDs having a heterojunction, it is to be understood that the materials and methods described herein may also be used in OLEDs that do not contain a heterojunction. Such heterojunction-free OLEDs are sometimes referred to as single layer devices.
While the preferred embodiments of the present invention have been described as having a second ETL, which is present between the phosphorescent-doped first ETL, and the cathode, the present invention is also directed to a phosphorescent-doped host ETL that is in direct contact with the cathode. In this embodiment of the invention, the host ETL is still required to have a triplet excited state that is of higher energy than the emissive triplet excited state of the phosphorescent dopant, such that there are no significant non-radiative losses via energy transfer through the triplet excited state of the host.
In yet another embodiment of the invention, the host ETL is still required to have a triplet excited state that is of higher energy than the emissive triplet state of the phosphorescent dopant. However, rather than requiring the HOMO levels of the phosphorescent dopant molecules to function as hole trapping sites, a second ETL is present that functions as a hole blocking layer between the phosphorescent doped ETL and the cathode.
A hole blocking layer (HBL) comprises a material, typically an electron transporting material, that is effective in blocking transport of holes through the HBL. Such transport of holes can lead to significant loss of device efficiency due to the undesired recombination of holes and electrons at the cathode surface. An effective hole blocking material is one for which the ionization potential of the material is at least slightly larger than the ionization potential of the adjacent emissive layer. For example, specifically, the hole blocking material has an ionization potential at least about 0.1 eV larger than the ionization potential of the adjacent emissive layer. Such materials of the hole blocking layer also preferably have high electron transporting properties, such as a high electron mobility.
Thus, in these representative embodiments of the present invention, the invention may be characterized as relating to OLEDs that simply include a phosphorescent-doped host ETL in which the host ETL has a triplet excited state that is of higher energy than the emissive triplet state of the phosphorescent dopant.
It is to be understood that, while the host ETL may typically have more than one triplet excited state, each triplet excited state is required to be of higher energy than the emissive triplet excited state of the phosphorescent dopant. Thus, reference herein to the triplet excited state of the host ETL that is required to have a higher energy level than the energy level of the emissive triplet excited state refers to the lowest triplet excited state of the host ETL, if there is more than one triplet excited state in the host ETL.
Further objectives and advantages of the present invention will be apparent to those skilled in the art from the detailed description of the disclosed invention.